1. Field of the Invention
This invention relates to whiskers of zinc oxide having a hitherto unknown crystalline structure or form which has at least two needle-like crystals radially extending from a central part or body. The invention also relates to a method for making whiskers having such a novel form as mentioned above. The whiskers of the novel form or structure have wide utility as additives in various fields of electrophotography, electronic parts such as varisters and ferrites, a vulcanization accelerator for rubbers, fillers or pigments for paints, inks, synthetic resins, medicines, paints, cosmetics, ceramics and the like. In addition, the whiskers may be utilized as a reinforcing agent for metals, ceramics, resins and the like and also as a filter material, a carrier for catalyst, humidity and gas sensors, an electromagnetic shielding material, an electric conductor and the like.
2. Description of the Prior Art
Zinc oxide currently employed as a general-purpose industrial material is one which is produced by a so-called French process. The zinc oxide obtained by this process consists of particles having different shapes and sizes.
Japanese Patent Publication No. 60-5529 describes a process of forming needle crystal particles of zinc oxide in high yield. This process is an improvement of the French process in which metallic zinc vapor is quenched, in which fine needle crystals having a length of from 0.5 to 1.5 micrometers are obtained without formation of large-sized crystals. The needle crystals of zinc oxide are smaller in size in about two orders of magnitude than other industrially employed whiskers of other materials commerically available at present. Usually, whiskers have commonly a great effect of reinforcing metals, ceramics and resins. However, the needle crystals of zinc oxide as mentioned above exhibit only such a low level as ordinary zinc oxide particles with respect to the reinforcing effect, and are thus unsatisfactory as whiskers. More particularly, whiskers which are made of fibrous single crystals have significantly greater mechanical strength than crystal particles for the same material. This is the reason why whiskers have attracted attention for use as a reinforcing material in order to attain high mechanical strength by incorporation into other materials. At present, whiskers for industrial purposes such as of metals, metal oxides, metal carbides, metal nitrides and the like have been commercially sold.
However, these known whiskers have several problems. One of the problems is that because of a small growth rate, the production costs become high. Another problem is that since the whiskers have a simple fibrous form, it is very difficult to uniformly disperse the fibrous whiskers three-dimensionally in matrix materials such as resins or metals. When the dispersion is formed or shaped, the whiskers are liable to be arranged along the direction of melt flow of the matrix. The resultant composite material eventually exhibits anisotropy in characteristics.
Whiskers of zinc oxide having a length in the order of millimeters are set out in Japanese Laid-open Patent Application No. 50-6597. These whiskers are produced from zinc alloys with the following disadvantages: the whiskers inevitably contain impurities in the crystals and have to be grown on a substrate; the yield is low; and apparatus and operations for making the whiskers are complicated with a long-term heating treatment being required. Thus, the whiskers are obtained only experimentally rather than industrially.